Level structure and transition multipolarities of 54Mn

Excited states up to 3 MeV in the odd-odd 54Mn have been populated in the 54Cr(p,n)54Mn reaction at Ep = 4.5 MeV. Conversion coefficients (αK and απ ) were deduced using the NPG method for the first time in 54Mn. Multipolarities were unambiguously assigned in 16 transitions for the first time using the obtained conversion coefficient values. Spin and parity has been restricted for three transitions.


Introduction
Nuclei around doubly magic 40 Ca and 56 Ni can manifest valence nucleon configurations, collective excitations, super-deformation, and shape coexistence.The odd-odd nuclei in the vicinity of Z = N = 20 and 28 shell closure present a unique opportunity for testing the underlying protonneutron residual interaction, which is believed to drive the emergence of collective excitations.These nuclei exhibit a complex level structure due to many possible couplings of unpaired nucleons to the even-even core [1].The properties of the low-and high-spin states in oddodd 54 Mn (Z = 25, N = 29) have been studied [1,2,3] via different probes.Spectroscopic information such as γ-ray branching ratios, and multipole mixing ratios, were determined for transition energies, 54 keV up to 1509 keV (from J π = 2 + up to 6 + ) [4,5].In Kumar et al., [2], the excited states in 54 Mn were populated using the 51 V( 20 Ne,xn, yp, zα ) 54 Mn reaction up to excitation energy of 5 MeV, J π = 15 + .However, the information on transition multipolarities in 54 Mn is still scarce.This article reports on the first conversion electron and electron-positron pair conversion study of 54 Mn.Conversion coefficients have proven to be useful tools in the determination of multipolarities and spin-parity assignments [6,7].

Experimental details
The electromagnetic decays of excited states in 54 Mn were observed in the 54 Cr(p, n) 54 Mn reaction at 5.4 MeV bombarding energy using proton beam from the Heavy Ion Accelerator Facility (HIAF) at the Australian National University.Conversion electrons and electronpositron pairs were measured with an array of six 9-mm thick Si(Li) detectors with 2.5-keV energy resolution called Miel [8].A schematic view of the Super-e spectrometer with other details is given in [9].The Miel detectors can be operated in singles mode, allowing the measurement of the conversion electron spectrum.Alternatively, to measure electron-positron pairs, they can be operated in the sum-coincidence mode by summing all 15 combinations of detector pairs.The spectrometer efficiency as a function of electron energy was calculated using the LensIPF code [8] and normalized to the relative efficiency of the Miel detectors measured using an in-house produced 170 Lu source.The data points were obtained from a 170 Lu source measurement (squares with error bars) compared to the calculated efficiency using the LensIPF Monte Carlo code (red curve) [8].
Gamma rays were detected simultaneously with the conversion electrons and electron-positron pairs by making use of a high-purity Ge (HPGe) detector located at 150 cm from the target.The HPGe relative γ-ray detection efficiency was measured using 152 Eu and 56 Co sources.

Result and discussion
The γ-rays singles, ICE and IPF spectra of the observed transitions from the 54 Cr(p, n) 54 Mn reaction are presented in Fig. 2. The solenoid current was swept between 4.830 A to 8.593 A, allowing the observation of conversion electrons (CE) in the 900 keV -2500 keV energy range.

Conversion coefficients
The internal conversion (α K ) and pair conversion (α π ) coefficients for transitions in 54 Mn were determined through the so-called Normalized Peak to Gamma (NPG) method [10].The method involves the use of relative efficiencies for detected γ rays (η γ ), CE (η CE ) or electron-positron pairs (η π ), and a normalization factor which is determined using a well-known experimental or theoretical conversion coefficient of a reference transition.It is customary to use the strongest transition with a well established multipolarity as a reference transition.When precision measurements are required, especially in a case where the multipolarity is well established [10], the NPG method is preferred over alternatives [11].The method is described extensively in [10,11].The experimental and theoretical internal conversion coefficients (ICC) are qualitatively compared in Fig. 3.The 1784.8 keV 1 + → 3 + (E2) transition was used for normalization.
Using CE and electron-positron spectroscopy, a number of conversion coefficients have been determined for the first time.The deduced conversion coefficients restrict the possible spin-parity for some states that were previously more uncertain or unknown [12].Three transitions have experimental ICC values above the theoretical E2 transitions, however, there is not sufficient experimental evidence to support the presence of an E0 component as the error bars overlap the E2 line.In addition, due to spin-parity selection rules, no E0 strength is possible for 2 + → 3 + or 1 + → 2 + transitions.Therefore, only the 1579.4keV (2 + → 2 + ) transition is a candidate for E0 and will be evaluated for ρ 2 (E0) strength in our subsequent publication.The newly assigned multipolarities and new restrictions on the spin-parity of three transitions are shown in Table

Conclusion and outlook
The enhancement in the ICC value of the 1579.4keV transition is indicative of the presence of an E0 component.However, the mixing ratio δ(E2/M 1) is unknown.Therefore, to probe for E0 strength, plans are underway to perform angular distribution measurements using the same reaction.The lifetime of the state will also be re-measured.

Figure. 1 Figure 1 .
Figure 1.Relative detection efficiency of the electron spectrometer in the lens mode.The data points were obtained from a 170 Lu source measurement (squares with error bars) compared to the calculated efficiency using the LensIPF Monte Carlo code (red curve)[8].

Figure 2 .
Figure 2. (a) Singles gamma ray and (b) CE spectra.The CE spectrum was shifted by the K-shell binding energy to align the peaks with the E γ energy.(c) Pair spectrum measured with stepped field and by 2m 0 c 2 energy to reflect the transition energy. 1.